New particle hopes fade as LHC data 'bump' disappears
By Pallab Ghosh
Science correspondent, BBC News
5 August 2016
From the section Science & Environment
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Media captionDisappointment: Cern's Dr Marco Delmastro speaks to Horizon on BBC2
Hopes for the imminent discovery of a particle that might fundamentally change our understanding of the Universe have been put on hold.
Results from the Large Hadron Collider show that a "bump" in the machine's data, previously rumoured to represent a new particle, has gone away.
The discovery of new particles, which could trigger a paradigm shift in physics, may still be years away.
All the latest LHC results are being discussed at a conference in Chicago.
David Charlton of Birmingham University, leader of the Atlas experiment at the LHC, told BBC News that everyone working on the project was disappointed.
"There was a lot of excitement when we started to collect data. But in the [latest results] we see no sign of a bump, there's nothing.
"It is a pity because it would have been a really fantastic thing if there had been a new particle."
Speaking to journalists in Chicago at the International Conference on High Energy Physics (ICHEP), Prof Charlton said it was a remarkable coincidence - but purely a coincidence - that two separate LHC detectors, Atlas and CMS, picked up matching "bumps".
"It just seems to be a statistical fluke, that the two experiments saw something at the same mass.
"Coincidences are always strange when they happen - but we've been looking very hard at our data to make sure we fully understand them, and we don't see anything in the new sample."
'New Physics Island'
Prof Fabiola Gianotti, director general of Cern, which runs the LHC, said her team was "just at the beginning" of a journey and emphasised the collider's technical success.
"The superb performance of the LHC accelerator, experiments and computing bode extremely well for a detailed and comprehensive exploration of the [new] energy scale, and significant progress in our understanding of fundamental physics," she said.
view inside the Atlas experiment at CernImage copyrightCERN/SCIENCE PHOTO LIBRARY
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The LHC's Atlas detector was one of two experiments that got a hint of a new particle
Prof Jon Butterworth from University College London, who also works on the Atlas experiment, said that he and his fellow researchers will now have to work a lot harder to discover a family of new particles that they believe must be out there.
"If you imagine we have landed on a new island of physics and we are scanning the landscape - if we don't see anything this year, it means that there are no amazing new civilisations with huge cities; there are no spectacular volcanoes there that we see with our first flyby.
"But it doesn't mean that there is not something hiding in the undergrowth that we will find later. It just means that we will have to do that slowly and carefully and really have to do our job over the next months and years."
iWonder: What will the LHC discover next?
LHC: The quest to 'break physics'
Scientists at the LHC discovered the Higgs Boson four years ago. That particle was a long-standing prediction of the current theory of subatomic physics - the Standard Model.
Their aim since then has been to search for evidence of phenomena beyond that model.
The Standard Model elegantly explains how particles combine and interact to create the world around us. It also explains how the forces of nature such as electricity and magnetism work, as well as how nuclear forces operate to hold atoms together.
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These elementary particles make up the world around us, according to the Standard Model
But for all its successes, physicists now know that the model explains a tiny corner of our existence - just 4% of the Universe.
It does not explain how gravity works, and it does not cover dark matter - the mysterious substance that accounts for more than a quarter of the Universe. It also fails to explain dark energy, which accounts for some two-thirds of the Universe.
Seeking a revolution
The LHC was built to explore those unknowns and search for new physics, in the hope of developing theories that would explain how the bulk of the Universe works. Any such discovery would herald the biggest revolution in physics for a hundred years.
Hints of a new particle last December - from CMS and Atlas, two of the main detectors at the LHC - generated huge excitement in the particle physics community.
Infographic
There was a "bump" in the data which could have been a statistical fluke - but it just might have been the first evidence of the new physics researchers had been waiting for.
Earlier this year, the revamped LHC finally reached its full operating potential and was set to collect data from collisions at an unprecedented rate.
Thousands of scientists at Cern were hoping that they might be on the verge of one of the all-time biggest discoveries in science - even bigger than the Higgs.
But despite knocking on the portal to a new realm of physics harder than they had ever knocked before, they found that the door was still firmly shut.
The experiment wore on and the bump disappeared. Irregularities like this come and go in particle physics, so it didn't come as a huge surprise to the scientists, but there was obvious disappointment.
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Inside Cern
As well as having analytical minds, these physicists naturally also have passionate, beating hearts. Their disappointment was palpable according to Thomas Hewitson, an assistant producer for BBC2's Horizon programme.
Thomas and his colleagues filmed the LHC researchers during those tantalising few months.
"When we first visited in May there was a real buzz about the 'bump'," he said.
"They're scientists, so they knew it was too early to pop the champagne, but even the most cautious among them went giddy when they allowed themselves to imagine a brand new particle. They hoped they were re-writing the textbooks.
"So when they saw that the bump had gone... they looked devastated. They are good experimentalists, and so they had always said it might be nothing. But they're only human, and the disappointment was obvious."
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Media captionDr Livia Soffi speaks to Horizon about the initial "bump"
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So how long before the LHC discovers new physics? According to Prof Charlton, if scientists don't see something soon, they may have to settle in for the long haul.
I hate to describe supersymmetry as a 'zombie theory'... but every time we look and we don't find any obvious sign of it then it becomes a little less plausible
Prof Jon Butterworth, University College London and LHC
"If we don't see new particles this year we may need to run for quite some more years to get significant observations of new particles," he told BBC News.
In that event, the team will have to look for more subtle indications than an obvious bump in the data - which, Prof Charlton said, was "almost too good to have been true".
"Most of the models for new physics predict rather more complex things. It is not likely to be that easy and we'll have to look for more complex signatures then it can take a lot longer".
If new particles will be hard to find, then one popular theory called supersymmetry looks to be in trouble. The idea, cherished by many theoretical physicists, is that the new particles the LHC has yet to discover are heavier, "super" versions of the ones we know about.
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Media captionIn search of new physics
According to Prof Butterworth, it's a theory that refuses to die.
"I hate to describe it as a 'zombie theory' but it is never going to be absolutely ruled out. But every time we look and we don't find any obvious sign of it then it becomes a little less plausible."
Another possibility is that the existing Standard Model might have more life in it, he added.
"The theoretical arguments to say that it does not work [in the realm of new physics] might be flawed and we might have more to understand just from the Standard Model."
The LHC has taken physicists into a new territory and a paradigm shift in our thinking is still very much in the offing. But it is going to take a little longer than many had hoped.

物理學家發現的新重子，在理論上存在長久，但此前從未觀測到。重子由夸克組成，質子和中子是最常見重子。在自然世界中，重子頂多僅含1個重夸克，新發現的重子則含有2個重夸克LHCb announces a charming new particle 外電
Today at the EPS Conference on High Energy Physics in Venice, the LHCb experiment at CERN’s Large Hadron Collider has reported the observation of Ξcc++(Xicc++) a new particle containing two charm quarks and one up quark. The existence of this particle from the baryon family was expected by current theories, but physicists have been looking for such baryons with two heavy quarks for many years. The mass of the newly identified particle is about 3621 MeV, which is almost four times heavier than the most familiar baryon, the proton, a property that arises from its doubly charmed quark content. It is the first time that such a particle has been unambiguously detected.
Nearly all the matter that we see around us is made of baryons, which are common particles composed of three quarks, the best-known being protons and neutrons. But there are six types of existing quarks, and theoretically many different potential combinations could form other kinds of baryons. Baryons so far observed are all made of, at most, one heavy quark.
“Finding a doubly heavy-quark baryon is of great interest as it will provide a unique tool to further probe quantum chromodynamics, the theory that describes the strong interaction, one of the four fundamental forces,” said Giovanni Passaleva, new Spokesperson of the LHCb collaboration. “Such particles will thus help us improve the predictive power of our theories.”
“In contrast to other baryons, in which the three quarks perform an elaborate dance around each other, a doubly heavy baryon is expected to act like a planetary system, where the two heavy quarks play the role of heavy stars orbiting one around the other, with the lighter quark orbiting around this binary system,” added Guy Wilkinson, former Spokesperson of the collaboration.
Measuring the properties of the
Ξcc++ will help to establish how a system of two heavy quarks and a light quark behaves. Important insights can be obtained by precisely measuring production and decay mechanisms, and the lifetime of this new particle.
The observation of this new baryon proved to be challenging and has been made possible owing to the high production rate of heavy quarks at the LHC and to the unique capabilities of the LHCb experiment, which can identify the decay products with excellent efficiency. The Ξcc++ baryon was identified via its decay into a Λc+ baryon and three lighter mesons K-, π+ and π+.
The observation of the Ξcc++ in LHCb raises the expectations to detect other representatives of the family of doubly-heavy baryons. They will now be searched for at the LHC.
This result is based on 13 TeV data recorded during run 2 at the Large Hadron Collider, and confirmed using 8 TeV data from run 1. The collaboration has submitted a paper reporting these findings to the journal Physical Review Letters.
Find out more:LHCb updateEPS conference (link is external) websiteSlides presented at the EPS Conference on High Energy Physics